Why did my battery explode? Bring it….

[pstadt]

I have three house batteries on our boat (which is winterized and in the water) and have kept the charger/inverter on. There is minimal draw - LEDs on the breaker panel, digital clock on the radio, etc. Batteries have been registering "full" on every periodic check of the boat, even today (01/18/14). Batteries are going on three years old this spring.
While checking the starboard engine compartment today, found the aft battery of the three had exploded, with no apparent flames or damage, other than acid on most everything. See pic.
From what I can tell, they are a European brand AGM 12V120a (20hr). The remaining batteries appear OK. I've isolated the bank and shut down the inverter until I can clean up and replace the ex-battery. I wiped up everywhere (did in a sleeve on a favorite nylon jacket in the process) and got the battery ready to pull.
We did have a sharp cold spell about two weeks ago, single digit (F) for about two days, followed by a warm spell (55F). Back down to freezing temps again now. I've watched videos on how AGMs are built and operate and one on checking the voltage individually on banked batteries but couldn't find any on what causes the type of casualty we experienced today.
So, jokes and pokes are welcome, but please offer a real conclusion as to why this may have happened.

Thanks!

Pat